Generally, in a conventional phase conduction motor, a core made of a magnetic material, such as laminated silicon steel plates, occupies most of the weight of a rotor, and a field and an armature coil are inserted into a slot having the core. Meanwhile, a superconducting motor uses a superconducting field winding which generates a strong magnetic field, thus generating the same output as that of the conventional phase conduction motor with the size of ½ to ⅓ of that of the conventional phase conduction motor without using the core in the motor.
In order to obtain a desired output in the air-cored structure that does not use the core, a field coil is placed in a rotating cryostat, so that a strong magnetic field is formed under a superconducting condition. Thus, devices for cooling a superconducting coil must be placed together with electrical coils. In the case of a field coil made of Bi2223 oxide-based superconducting wires which are not used in existing rotary machine technology and have been currently used in cryotechnology, the field coil is cooled to about 30K, and liquid neon or helium gas is mainly used as a refrigerant.
The superconducting motor/generator is advantageous in that a strong magnetic field can be generated in the superconducting field coil, so that size and weight are remarkably reduced in comparison with a conventional machine, and efficiency can be increased. However, the superconducting motor/generator is disadvantageous in that the superconducting field coil must be cooled to a very low operating temperature of 50K or less. Further, even if a stationary superconducting magnet, such as an MRI, uses a metal-based superconducting wire which must be cooled to 4.2K, technical difficulties are absent because of the development of cryogenic cooling technology. However, in order to supply a refrigerant to the rotating superconducting field coil, such as a superconducting rotary machine, very low temperature must be maintained in a stationary cryo-cooler in which the refrigerant is liquefied and also in a duct through which the refrigerant flows in and out, and a part for coupling a rotary part to a stationary part is required. In order to solve this problem, a Ferro-fluid seal is installed. However, the coupling part is complex in structure and is weak, so that reliability becomes deteriorated when the motor/generator has been operated for a lengthy period of time, and cooling efficiency becomes deteriorated. Thereby, the entire efficiency of the superconducting motor/generator is reduced.
Since the rotating cryogenic superconducting field coil usually generates a DC magnetic field, it is used as the field coil of a synchronous machine or a DC machine. A phase conducting copper coil which has been used in an existing motor is used in an armature winding in which an AC magnetic field is generated.
Thus, most superconducting machines which have been currently developed are constructed so that a magnetic field shield is provided on the outermost layer of the machine by layering a ferromagnetic body applied to the existing rotary machine in a cylindrical form, and a three-phase conduction coil is arranged inside the mechanical shield.
A warm damper manufactured using aluminum or copper and having good electrical conducting ability is installed between the armature winding and the superconducting field coil, flows inducing current when the existing synchronous machine is stepped out, thus aiding in recovering a synchronous speed. Further, the warm damper serves to prevent an AC magnetic field generated in the armature winding from affecting the superconducting field coil generating a DC magnetic field. A cryogenic damper placed between the cryostat and the superconducting field coil serves to shield radiant heat transferred from an outer covering of the rotor. Generally, the warm damper is used when using the oxide-based superconducting wires having high invariability.
Since the superconducting motor/generator currently developed has a rotary field structure, the problems of the cryogenic cooling system of the superconducting machine, that is, the complex structure for cooling the rotating superconducting field coil, the deterioration of reliability and the reduction in cooling efficiency due to long operation must be overcome. That is, improving the cryogenic cooling system of the superconducting rotary machine is required. Further, when the superconducting field coil has an air-cored configuration in a middle or small machine of about 10 MW, too many superconducting wires which are expensive are required, so that the economic efficiency of the superconducting motor/generator is low.